ECRI Microelectronics

Study on Erythema of Gold Film on LTCC Substrates

Study on Erythema of Gold Film on LTCC Substrate
Analysis and Research on Golden Film Layer with Red Spot in the Surface of LTCC Substrate

Abstract: This paper mainly studies the problem of golden film with red spot in the surface of LTCC substrate. Scanning electron microscopy (SEM)and X-Ray photo electron spectroscopy( XPS) were used as the analytical methods. The micro structure and composition of the red spotted area on LTCC golden film layer are analyzed by scanning electron microscopy. It is found that the red spotted area contains a small amount of Ag element. Meanwhile, the state of binding energy of the outer electrons of the atomic atoms in the red spots on the surface of the LTCC substrate is observed and analyzed. Besides, it is found that the red spotted area contains a trace amount of sulfur in addition to small amount of silver. Through further research, the chemical bonding state of silver and sulfur was analyzed, and the composition of the red spotted area on the surface of the gold film of the LTCC substrate was determined to be a mixture of silver sulfide and silver oxide, which explain the formation mechanism of the red spot and propose corresponding prevention and improvement measures. Finally, a series of reliability tests and demonstrations were carried out on the substrate with red spot and fading treatment. The results show that both layers are qualified, which proves the effectiveness and feasibility of the speckle removal measures.
Key Words: LTCC circuit substrates, golden film layer, red spot phenomenon, X-Ray photo electron spectroscopy (XPS), mechanism, speckle removal treatment

Preface
LTCC (low temperature co-fired ceramic) technology (Low Temperature CofiredCeramics, LTCC) is that that low-temperature sinter ceramic powder is made into a raw ceramic tape with precise thickness and density, which is used as a circuit substrate material, and the circuit pattern needed is made on the raw ceramic tape by the processes of punching, filling and conductor printing, and a plurality of passive components are embedded in the raw ceramic tape, and then are laminated together and fired at 850 DEG C
The passive integrated module of the three-dimensional circuit network can also be made into a three-dimensional circuit substrate with passive components, on which IC and active devices can be mounted, and the passive/active integrated microwave T/R functional module can be made.
LTCC substrate has the following advantages: 1) the use of raw ceramic lamination method, the number of substrate layers can be relatively unlimited; 2) the substrate material added glass phase, sintering temperature is low (850 ℃); 3) the use of low square resistance conductor (Au, Pt-Pd-Au, Pd-Ag, Ag, etc.) To print microstrip lines, greatly improving the wiring density and signal transmission speed; 4)The thermal expansion coefficient of the substrate is close to that of the silicon device, which is very favorable for mounting the bare silicon device; 5) The LTC substrate can be embedded with passive components to form a three-dimensional high-density module.
In the process of LTCC circuit board production and development, especially after the substrate is placed for a period of time, the surface of the gold film layer on the substrate surface usually appears some color red or red spots phenomenon, the problem mainly appears in the top layer of the substrate and the gold conductor of the Bot layer.In this paper, the phenomenon of red spot on the surface of gold layer is studied, the composition and formation reason of the spot on the surface of gold layer are analyzed, the preventive measures are put forward, the surface spot is removed by means of heat treatment, and the reliability test is carried out in the repaired area of gold layer, and an effective scheme for removing the red spot on the surface is determined.

2 Experimental Design
2.1 Preparation of Experimental Samples
In daily production practice, the phenomenon of red spot on the surface of gold film layer mainly occurs on the surface of Femo system substrate products. Therefore, the A6M type raw porcelain slice of Fero system is selected as raw material in this experiment, and the raw porcelain slice is printed with matching gold paste on the surface layer and with matching silver paste on the middle layer by means of screen printing, and a batch of test samples are prepared after a series of LTCC substrate processing technology treatment.

After the qualified samples are packed and placed in the packaging box for a period of time, red spots can be found on the surface of local areas of the gold film layer of some samples, as shown in Figure 2 (a) and (B) below:

2.2 Analytical instruments and equipment
The red spots on the surface of LTCC gold film were observed by A60H high power microscope produced by Leica company in Germany, and the maximum magnification was 50 times. The morphology and composition of the red spots on the surface of ITCC gold film were analyzed by GeminiSEM500 scanning electron microscope produced by Carl Zeiss company in Germany, and the binding energy of the outer electrons of the red spots on the surface of LTCC gold film were observed and analyzed by ESCALAB250 X-ray photoelectron spectroscopy (XPS) made by Thermo-VG Scientific company in America.
A series of reliability tests were carried out on LTCC substrates with erythema on the surface of gold film, and the film properties, including adhesion, shear force and gold wire bonding tension, were tested and analyzed before and after the reliability tests.

Experimental results and analysis
3.1 Selection of Experimental Samples
In this analysis, two LTCC substrates were selected, numbered 1 # and 2 #: 1 # substrate: LTCC substrate with serious erythema on the surface, 2 # substrate: LTCC substrate with normal erythema on the surface.
3.2 Experimental process and scheme design
In order to analyze the composition of the red spots on the gold film surface of LTCC substrate, we designed two groups of experiments, the specific experimental scheme is as follows:
The first group: SEM was used to analyze the morphology and composition of the red spot area and non-red spot area on the gold film surface of LTCC substrate.
The second group: X-ray photoelectron spectroscopy (XPS) was used to analyze the characteristic spectral levels of element atoms in the erythema region on the surface of gold film on LTCC substrate, and to study the chemical binding state of erythema components.
3.3 Experimental results
3.3.1 Analysis and Results of SEM Experiments
The microscopic morphology of the erythema region on the surface of the sample was observed by SEM. Fig. 3 (a) and (B) are the microscopic morphology of the erythema region of the sample with magnification of 500 and 2000 times, respectively.

It can be found from the figure that there are a large number of micro-holes between gold particles on the surface of LTCC substrate, the shape of the holes is mainly irregular arc, and the pore size is concentrated between 2 ~ 5.5.The formation of a large number of holes on the surface of LTCC substrate is mainly caused by two reasons: (1).When the substrate is sintered in the discharge furnace, the substrate surface is sintered first, and a sintered film is formed on the substrate surface. When the substrate is sintered at a high temperature, the raw porcelain slice and the metal conductor paste (including conductor pastes such as Au, Ag, PbAg, PtPbau, etc.) As well as the solid organic component contained in the resistance paste will decompose to form a gas under a high temperature in the discharge furnace, which will volatilize through the gold film layer on the substrate surface, thereby forming micropores in the gold film layer on the substrate surface.When the glass is melted, the materials such as H2BO3, Na2CO3, Na2SO4 and NaNO3 will decompose and release a large number of gases, such as CO2, SO2 and NO2.
By observing the microscopic morphology of the speckle region, it is found that there is a large color difference between the speckle region and the surrounding normal region, as shown in Figure 3 (a). To determine the composition of the red speckle, the speckle region and the normal gold layer region are analyzed by scanning electron microscopy (SEM), and the scanning region is shown as the rectangular region in Figure 3 (B). Figure 4 (a) and (B) are the EDAX electron spectra of the red speckle region and the normal region of the substrate, respectively.


From the composition spectrum of SEM, we can see that in addition to gold element, a small amount of Ag element is also detected in the speckled area on the surface of the gold layer of the substrate. In addition to gold element, there are also LTCC ceramic functional phase elements Al, Ca, Si and O in the detection results of the gold layer in the normal area. The composition elements and contents of the erythema area and the normal area of the substrate are shown in Table 1.


During the sintering process of LTCC substrate, the internal gas is discharged and many micropores are formed on the surface of the gold film layer. At the same time, the ceramic functional phase material in the molten state in the ceramic substrate of LTCC substrate will diffuse to the surface of the gold layer through these micropores of the surface gold film layer, so A, Ca, Si and 0 elements can be detected in the erythema region and non-erythema region on the surface of the gold layer.
It can be seen from Table 1 that the surface erythema area of gold film on LTCC substrate has more Ag element than the normal area without erythema. Therefore, it can be speculated that Ag element is the root cause of the formation of erythema on gold layer surface of LTCC substrate, and the erythema on gold layer surface of LTCC substrate may be some substance formed by the reaction of this part of silver.
When the content of these elements is less than 1%, they may not be detected by analysis. The percentage content of these elements combined with Ag in the surface erythema of gold film on the substrate is extremely low, so the chemical combination with Ag can not be detected in the SEM composition analysis.Energy spectrum and composition analysis have been repeatedly verified, this limitation is very unfavorable for us to understand the formation mechanism of red spot on the surface of gold layer.
In order to study the formation mechanism of the surface erythema of the gold film on the substrate, another surface analysis method, X-ray photoelectron spectroscopy (XPS), is used to analyze the surface of the gold film. The depth of the surface analysis is ~ 8mm, the size of the light spot is 500μm, and the resolution is 1%.
3.3.2 X-ray photoelectron spectroscopy (XPS) experimental analysis and results
Table 2 and Table 3 are the results of X-ray photoelectron spectroscopy (XPS) composition analysis of the red spot area of gold layer on the surface of sample 1 and the surface of gold film on sample 2, respectively.

Table 2 and Table 3 are compared with Table 1, two conclusions can be obtained as follows: (1) The binding energies of Au on the surface of 1 # sample and 2 # sample are 83.70 eV and 83.74 eV, respectively, and the difference between the two values is only 0.04 eV, which is very close.Therefore, the composition of the erythema formed in the gold film on the LTCC substrate surface can be determined not to be oxide of Au oxidation or other forms of Au compounds, thus excluding the possibility that the erythema is due to the reaction of Au itself. (2) A small amount of S element (2.58 at%) was newly found on the surface of the erythema region of the 1 # test sample.

4 Analysis and Discussion
4.1 X-ray photoelectron spectroscopy
Compared with Table 2 and Table 3, the composition of sample 1 # is more silver and sulfur than that of sample 2 #. The specific conditions are as follows: Ag3d5/2, the corresponding binding energy level is 367.76eV, atomic percentage content is 2.13at%; S, the corresponding binding energy level is 161.40e, atomic percentage content is 2.58a%. The X-ray characteristic photoelectron spectra of Ag and S in the erythema region of sample 1 # are shown in Figure 4 and Figure 5 respectively.

Figure 4 corresponds to the binding energy of 367.76eV, which corresponds to the energy range of Ag atom in Ag2S (367.68eV ~ 368.43eV), and may contain a small amount of Ag2O.


At the same time, the spot region of "1" sample contains a certain amount of sulfur with binding energy of 161.40 eV, which corresponds to the energy state of S2p3/2 in metal sulfides according to the energy spectrum of S2p3/2. After analysis, this part of sulfur should be produced by the reaction of Ag in the spot region with sulfur components such as SO2 or H2S in the atmosphere.
The content of Ag on the surface of 2 # test sample is zero.
4.2 Mechanism of erythema formation
Ag reacts slowly with O 2, SO 2 and H 2S in humid environment to form Ag 2O and Ag 2S. Therefore, the gold film on the surface of the substrate does not look significantly different from that of the normal substrate before the Ag mixed in the substrate is produced.However, as the time goes on, the mixed Ag on the surface of the gold film gradually reacts with O2, SO2 and H2S in the air to form Ag2O and Ag2S, and the erythema begins to grow.
Ag element on the surface of the substrate is the fundamental reason for the formation of erythema on the gold surface in the later period.
4.3 macular regression measures
The chemical composition of the brown rust spot on LTCC substrate surface is Ag _ 2S or Ag _ 0. The chemical properties of Ag _ 2S and Ag _ O are that AgO begins to decompose at 250 ℃, and rapidly decomposes above 300 ℃, and As _ 2S begins to decompose at 300 ℃ C, which is consistent with the temperature range of 270 ~ 300 ℃ that we explored in the previous stage.
In view of this, we can use the way of heat treatment for defacement treatment, after a series of experiments, we use the best heat treatment conditions are: heating temperature: 450 ℃, holding time: 30 min.
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